r/serial-console: Enable getty on serial console

The *serial-console* Ansible role enables and starts a systemd service
unit to activate a console getty on the specified serial console device
(by default: ttyS0).  This is particularly useful for virtual machines,
allowing one to control them in absence of a graphical VM management
tool.

roles/serial-console/defaults/main.yml

@@ -0,0 +1,3 @@
+serial_console_tty: ttyS0
+serial_console_service: >-
+  serial-getty@{{ serial_console_tty }}.service

roles/serial-console/tasks/main.yml

@@ -0,0 +1,8 @@
+- name: ensure serial console getty service is enabled
+  systemd:
+    name: '{{ serial_console_service }}'
+    enabled: true
+- name: ensure serial console getty service is active
+  systemd:
+    name: '{{ serial_console_service }}'
+    state: started